There are two kinds of thinking. Normal and deep.
Normal is taking the directions you were given to go somewhere. Deep is when you push deep into a subject and truly understand it, you go well past where most people would think would be enough. You understand the traffic patterns and the entire city map, you know why you were given the directions you were given.
If you want to be a writer you have to be a deep thinker. The only way to make a setting feel real is to make it deep and wide, you won’t show but a small part of it but you can’t just think about that part.
A society is not a jumble of unconnected things all taking place at once, its a complex system where everything touches everything else. Morals effect clothing, but so does utility and the economy. You can’t get everything but you should aim to go deeper then the average reader, so deep that your reader will never see the bottom.
The same goes for your characters, think about what motivates them, why they have their mannerisms, what their biggest disappoint in life was.
Again you don’t have to show everything, showing the tip is enough.
One thing that people think about science fiction is that you have to be an expert in engineering and physics to write it. You don’t, you just need to know where the stoplights are are.
By stoplights I mean things you can’t do, or if you do you must do with forethought.
As for the laws of physics there are really only a handful you need to be wary off. This is not saying you can’t violate them, but if you do it will have a big impact on your setting and you should think about it.
The first of course is faster then light travel. I will say that technically are a few ways around this that do not violate the laws of physics, but they tend to require exotic matter (matter with negative mass) or something equally unlikely.
Also be aware that you would be hard pressed to find a way to go faster then light in the real universe that would not also work as a time machine.
The second of course is time travel. Violating causality is probably the first or second thing most scientists would say if you asked them for a list of impossible things. No plot element causes more problems, for the love of god if you use this think it out.
The third is conservation of energy and momentum. Also known as not getting something for nothing. If something is being powered, where does it get the power? If something is moving what pushed it?
You don’t have to do math but if you have a machine doing something think about the kind of power it uses and what that means. If you have a ship move half the speed of light that takes a lot of power, it does not matter what kind of drive system it uses.
Lastly try not to go against the grain of science. By this I mean make your fake science SOUND like real science. Science in its most basic form will be the same in a thousand years as it is now, the thought processes will be the same because we will still be studying the same thing (the universe).
One trap you can fall into when placing your character in a setting is making creatures and settings so alien that they are unrelateable.
The first and most often given advise is for you to give your character problems that are familiar. While good advise this is not the only thing you can do. If you’re are going to do it however, the easiest way is to stick to the basics, food, water shelter, saving a loved one, basic primal urges that everyone has.
Once your story progresses you can and should expand, but make sure before you do that that your reader cares about your character.
You could also show that he acts very human, that he makes mistakes. Basically show that he thinks like a human, maybe contrast it with someone or something that does not.
Lastly you can do the opposite, make the reader hate the enemy of your character and root for your character just because he is the enemy of the bad guy.
This CAN work, but its hard to pull this off. And for the love of god give your character at least one or two good qualities.
By alien I mean different, so this could mean the future or more literal aliens.
Having an alien setting feel alien quickly is something you likely will have to do, never forget the first page of your book is by far the most important.
The first thing to keep in mind is that alien should not mean incomprehensible, it means different. Don’t go overboard too fast, always make sure there is enough understandable stuff for people to understand and relate.
You can start by changing simple things just enough to be noticed. For example the units of measurements, or the color or shape of things. If you describe a car as being spherical or say that your three plinks late for a meeting you establish that you are not in 21th century earth.
Another thing that you can try is to casually mention one of the big differences between them and us. You don’t have to describe it, just casually mention something that makes them different.
For the start of the book do that, mix the familuar with the new until the new is relateable.